In a conventional fabrication process of a semiconductor device, there has been used a plasma processing apparatus in which a semiconductor wafer (hereinafter, referred to as “wafer” for short) is etched by generating high density plasma at a relatively low atmospheric pressure. For example, in a parallel flat plate type plasma processing apparatus, a pair of parallel flat electrodes, i.e., an upper electrode and a lower electrode is placed inside a plasma chamber. Then, a plasma processing gas is introduced into the corresponding chamber, and, at the same time, a high frequency power is applied to one of the upper and lower electrodes to form a high frequency electric field therebetween. As a result, a plasma of the plasma processing gas is generated by the high frequency electric field to perform a plasma processing such as an etching on a wafer.
In such a plasma processing apparatus, a high frequency power for plasma generation and a high frequency power for bias, which is used for attracting ions of the plasma, overlap with each other and are applied to the lower electrode simultaneously, so as to perform an etching efficiently. Specifically, a high frequency power source for plasma generation is coupled to the lower electrode through a matching circuit (hereinafter, referred to as “matching box”) so as to apply the high frequency power for plasma generation, and, at the same time, a high frequency power source for bias is coupled to the lower electrode through the matching box so as to apply the high frequency power for bias, so that the two high frequency powers are overlapped with each other. The high frequency power for plasma generation is set to have a frequency of 100 MHz, taking a plasma generation efficiency into consideration. On the other hand, the high frequency power for bias is set to have a frequency of 3.2 MHz.
FIGS. 6A and 6B are graphs showing modulated wave spectrums of input and output in a conventional plasma processing apparatus, wherein FIG. 6A is for an incident wave and FIG. 6B is for a reflecting wave.
As shown in FIGS. 6A and 6B, harmonic components or modulated wave components corresponding to main frequencies of 100 MHz and 3.2 MHz, respectively, are developed during plasma generation in the plasma processing apparatus. The harmonic components or the modulated wave components appear in the vicinity of the main frequencies of the incident wave (Pf) and the reflecting wave (Pr), simultaneously. As a result, there occurs a malfunction of the matching box or the high frequency power source. Thus, the high frequency power source is provided with a power monitor for controlling the high frequency power by detecting the incident wave and the reflecting wave.
FIG. 7 is a schematic view showing an inner configuration of a power monitor in a conventional plasma processing apparatus.
As shown in FIG. 7, the power monitor has a directional coupler 300, a band pass filter 310 formed of an LC circuit, and a high frequency detector 320. The aforementioned harmonic components or the modulated wave components are removed by a detection circuit composed of the band pass filter 310 and the high frequency detector 320.
However, in the conventional high frequency power source having the above conventional detection circuit, the differences between frequencies of the main frequencies and the modulated wave components or the like included in the reflecting wave become small, if the difference between frequencies of the high frequency powers for plasma generation and bias corresponding to the incident waves becomes large. As a result, the modulated wave components or the like are not sufficiently attenuated by the band pass filter formed of the LC circuit, which may cause a malfunction of the high frequency power source since the modulated wave components are considered as the reflecting wave.
In addition, it is difficult to impress a proper high frequency power during plasma generation, in case where the modulated wave components or the like are not sufficiently attenuated.
Further in the conventional plasma processing apparatus, the reflecting wave (reflecting power: Pr) generated by the incident wave (incident power: Pf), which is outputted to the plasma chamber from the high frequency power source, is turned back from the plasma chamber by a variation in load impedance during plasma generation. Thus, in order to protect the high frequency power source from the corresponding reflecting wave, there has been employed a method for lowering the incident wave by detecting the reflecting wave or a method of providing a circulator between the high frequency power source and the plasma chamber to remove the reflecting wave. Particularly, the latter method is more efficient to insure an ignition margin during plasma generation, in case of the high frequency power source for plasma generation.
FIG. 8 depicts a block diagram showing a schematic configuration of a prior art plasma processing apparatus.
As shown in FIG. 8, a plasma processing apparatus 200 includes a plasma chamber 221 accommodating a wafer; a high frequency power source 226 for plasma generation; a matching box 222 made of a matching circuit for matching an input impedance of the plasma chamber 221 side with an output impedance of the high frequency power source 226 side such that the aforementioned reflecting wave becomes minimum; a power monitor 223 controlling a high frequency power by detecting an incident wave and a reflecting wave and supplying the combined power to the plasma chamber 221; a circulator 224 made of a magnetic material; and a dummy load 225 of a terminating resistance.
The high frequency power source 226 has a power monitor 227 performing the same function as the power monitor 223, and a plurality of power combiners 228 to 234 for combining and outputting powers. For example, the high frequency power source 226 outputs a high frequency power for plasma generation of 100 MHz and 3 kW as the incident wave.
The circulator 224 receives the incident wave Pf outputted from the high frequency power source 226 side and outputs pf′ to the plasma chamber 221 side, while outputting the reflecting wave Pr turned back from the plasma chamber 221 side to the dummy load 225. The dummy load 225 serves to dissipate the reflecting wave outputted from the circulator 224.
In the plasma processing apparatus 200, the high frequency power source 226 is protected from the reflecting wave by the circulator 224 that prevents the reflecting wave generated from the plasma chamber 221 due to a variation in load impedance during plasma generation from returning to the high frequency power source 226 directly.
However, it has been required a large space for the processing apparatus since the circulator 224 and the dummy load 225 for large power need to be disposed outside the high frequency power source 226, in the prior plasma processing apparatus 200.
Further, when the incident wave (Pf) from the high frequency power source 226 passes through the circulator 224, it is attenuated slightly (ΔPf=Pf−Pf′) due to a magnetic loss since the circulator 224 is made of a magnetic material. As a result, there has been a need for installing the power monitor 223 in front of the matching box 222 so as to precisely control the incident wave inputted to the plasma chamber 221 side.